The goal of this project is to identify the changes in the myocardium which are directly responsible for the development of irreversible ischemic injury. Experiments will be carried out in dogs in which myocardium is examined at various time intervals following occlusion of a coronary artery; in some animals reperfusion will be carried out and they will be sacrificed 7 days later when the necrotic and viable myocardium can be clearly differentiated. Both the ultrastructural appearance of the myocardium at multiple depths through the ventricular wall and the high energy phosphate concentrations will be correlated with the ultimate survival or death of the tissue, as determined by histologic examination. In other experiments, primarily using the isolated working rat heart, an attempt will be made to restore myocardial ATP concentrations by supplying exogenous purine nucleotide precursors at various time intervals following the onset of global ischemia. The point in the development of ischemic damage, as reflected in ultrastructural and biochemical changes, at which the myocardial cell is still able to synthesize high energy phosphates will be identified and it will be determined whether such synthesis is capable of maintaining or restoring ultrastructural integrity and mechanical performance. Critical to any study of the effect of ischemia on the myocardium is a determination of its severity. Accordingly, in the dog model, intramural partial pressures of O2 and CO2 will be measured in the ischemic zone, using a mass spectrometer and the observations will be correlated both with the ultrastructure and the high energy phosphate concentrations. It is hoped by these experiments, to identify the alterations in the process or processes responsible for irreversibility.